Urea Rate-limiting step

The rate limiting step In this side reaction Is the reaction of Isocyanate and water. The Importance of this reaction depends on the polarity of resin, the level and type of catalyst, and the humidity In the bake oven. The effect of this reaction Is to reduce the number of urethane crosslinks but Increase the number of urea crosslinks. The total crosslink density will likely decrease since two Isocyanate groups are necessary to form one urea link while only one isocyanate group is required to form a urethane link. If excess isocyanate groups are present, this reaction will consume the excess Isocyanate and Increase the crosslink density. This reaction can occur not only in the bake oven but also on aging of the coating if unreacted isocyanate groups are present after cure. 

Miscellaneous. The kinetics of the reaction of urea with the tetrakishydroxymethyl-phosphonium cation have been studied. The reaction involves the stepwise condensation of the hydroxymethyl groups with urea, and at least four different phos-phonium species are present in the reaction mixture. The proposed mechanism involves the dissociation of the initial cation as the rate-limiting step (Scheme 6). ... 

The reaction is driven forward by hydrolysis of pyrophosphate to inorganic phosphate. Argininosuccinate formation is considered as the rate-limiting step for urea synthesis. This reaction incorporates the second nitrogen atom of the urea molecule donated by aspartate. 

Available evidence suggests that the step catalyzed by arginine synthetase constitutes the rate-limiting step in the overall functioning of the urea cycle. As pointed out above, arginine catalyzes the conversion of arginine to ornithine. 

The authors suggested that the first rate-limiting step was an acid-catalyzed aldol condensation between the 1,3-dicarbonyl compound and the aldehyde affording aldol-type product 20. The following acid-catalyzed dehydration would furnish stabilized carbocation 21. Next, the urea would trap the carbocation 21 rendering the same intermediate 18, previous to the final cyclization to furnish Biginelli products 19. 

Under many conditions the rate-limiting step in urea synthesis is the carbamoyl phosphate synthase reaction. Isolated liver mitochondria will synthesize citrulline from added ammonia under appropriate conditions. The rate of citrulline synthesis can be shown to depend on the intramitochondrial content of acetylglutamate, and both the mitochondrial acetylglutamate content and the rate of citrulline synthesis increase on adding glutamate to the mitochondria. 

Carbamoyl phosphate is formed from CO2 and NH3 (or more accurately NH4+ as it exists at the pH within the body) in the mitochondrial matrix, catalysed by carbamoyl phosphate synthetase I, which is the rate limiting step of urea synthesis. A -Acetylglutamate regulates this step - this compound increases intrahepatically following ingestion of a protein-rich meal. 

A study of the kinetics of nitrosation of iV,iV -dimethyl-A"-cyanoguanidine in acid media (Scheme 13) [where the substrate exists as its conjugate acid (130)] has established that the mechanism of the reversible reaction is similar to that found for nitrosation of amides and ureas, rather than amines (for which attack of the nitrosating agent on the free base is usually rate limiting).The reaction, which is subject to general-base catalysis but not influenced by halide ion, involves reversible rate-limiting proton transfer in the final step and exhibits solvent deuterium isotope effects of 1.6 and... 

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